The invention relates to a method of refining the silicatic slag phase that is produced in the thermal recovery of phosphorus from raw phosphates, and the use of the refined material. The invention is especially suited for the refining of the glassy slag sand which is produced in a known manner when the molten phosphorus slag is quenched in water, and of which no really practical use has been made heretofore.
In German Auslegeschrift No. 1,804,172, a method is described for the preparation of synthetic wollastonite in which granulated glassy phosphorus furnace slag is roasted at 1000.degree. C. to 1100.degree. C. for one hour. The slag thus treated contains irregular crystal aggregates and has been identified as beta-wallastonite. This wollastonite is intended for use as a calcium raw material in ceramic compositions, so as to make it possible to obtain improvements in moisture expansion, expansion and contraction, and density.
The product obtained by the method of this Auslegeschrift is an opaque, gray-black material containing fine, black particles consisting in part of slagged reduction coke from the apatite reaction for the recovery of phosphorus, but also, and mainly, of black ferrophosphorus. If ceramic compositions are prepared using this material, consisting, for example, of 50% of the finely ground roasted slag and 50% clay substances, the plate produced from it will be "shadowy," i.e., it will contain dark discolorations. A perfect ceramic product cannot be produced in this manner. The described method of refining the phosphorus slag has actually never been adopted in practice to date.
In German Auslegeschrift No. 2,211,505 there is described a method for the refinement of the silicate slag phase produced in the thermal recovery of phosphorus from raw phosphate and quenched in water and granulated, in which the ferrophosphorus which granulates selectively in water is sorted out from the granular product. The sorting is accomplished especially by physical separation, for example by gravity separation with tetrabromoethane as the liquid phase. In this manner the silicatic raw slag phase from the recovery of phosphorus, which has an unsightly gray coloration, and which hitherto has been mostly dumped as a waste product or used as a cheap filler material in underground structures, is said to be converted to an improved product suitable for the preparation of wollastonite material and for the production of building materials.
In German Offenlegungsschrift No. 2,435,043 it is additionally proposed that material obtained by sorting the ferrophosphorus from the granulated silicatic slag phase be used as a raw material for glassmaking.
Extensive research into the production of synthetic wollastonite from quartz and calcium compounds by an incandescent heating process or by hydrothermal treatment followed by an incandescent heating process, is described in an article by A. Solf and W. Ohnemuller, "Darstellung von synthetischem Wollastonit aus Calciumhydrosilikaten," in Tonindustrie-Zeitung 94 (1970), No. 5, pages 197-214. The wollastonite material prepared by this method is unusable at the present time for reasons of cost.